When operating cryogenic equipment for low temperatures (less than 100 kelvin) or ultra low temperatures (less than 4 kelvin), there is often a need to change a sample at the cold part of the equipment. With conventional equipment using liquid cryogens such as helium or nitrogen, this is usually done by warming the equipment up and opening the equipment, or removing a part of the equipment and warming that up. The sample is then changed at room temperature. As this can be a slow process, some conventional cryogenic systems using liquid cryogens are fitted with more rapid sample change mechanisms that allow the majority of the system to remain cold. A key challenge with these systems is that the sample is entered into the equipment at room temperature, typically around 300K and then moved to another position where thermal contact is made with a body at a much lower temperature which in some systems can be lower than 1K. In systems using liquid cryogens the sample and associated mounting and connection equipment is usually pre-cooled either by passing it through cold cryogen gas on its way in to the system or by passing cold cryogen gas or liquid through the sample transfer mechanism, this reduces the thermal shock both on the sample and on the equipment.
More recently, cryogenic systems that do not require the addition of liquid cryogens or that only require liquid nitrogen during the initial cool down have been developed. These are generally known as cryogen-free systems. These systems use a mechanical cooler such as a GM cooler, Stirling cooler or a pulse tube to provide the cooling power. Because the cooling power of commercially available coolers is somewhat lower than the cooling power available from a reservoir of liquid cryogen, these systems can typically take longer to warm up, change the sample and cool down. There is therefore a considerable need for a method of changing samples in cryogen-free systems without the need to warm up the entire system.
With cryogen free systems there are a number of technical challenges when attempting to load a warm sample in to a cold cryostat. Firstly, the internals of the system are usually contained within a sealed vacuum vessel to reduce heat load. Secondly, within that sealed vacuum vessel, the sample space is usually enclosed by one or more radiation shields to further reduce the heat load. Thirdly, there are no liquid cryogens available to pre-cool the sample as it moves from room temperature to the cold mounting body. Also, electrical contacts need to be remotely made to the sample when it is loaded in the cryostat.
A number of these challenges are addressed in our earlier patent application WO2010/106309. In that application there is described a system in which a sample holding device is arranged to be coupled releasably via a thermal connector to one or more cold bodies within the vacuum chamber of the system so as to provide one or more stages of pre-cooling of the sample supported by the sample holding device. This apparatus is effective in providing staged pre-cooling of the sample prior to it attaining its operational or base temperature. Nevertheless, some challenges remain, particularly surrounding the need for extensive manual intervention in order to effect the sequential thermal couplings required to cool the sample.
For the case of cryogen cooling apparatus which comprises a dilution refrigerator system, any significant heat load which is applied rapidly to the system may cause a catastrophic failure in the relatively delicate components of the dilution refrigerator. There is therefore a need to provide automatic cooling and safe loading of samples, into cooling apparatus (particularly cryogen-free apparatus) which contains a dilution refrigerator for operating at ultra-low temperatures. It is these problems which the present invention has been devised to address.